CODEWITHSUNDEEP
c#image-processingpng
gzup
gzup

Reputation: 175

File size increases after reading png file from disk and saving it back

I'm currently working on a small program to read png files from disk, do some modifications and save it back. Everything is running smoothly except for one small problem, after I saved the file back to disk, its size always increases, for example, a 27.1MB file will become 33.3MB.

After some debugging I finally narrow it down to my reading and saving code. This is the code I'm currently using:

Bitmap img = new Bitmap(<path to file>);
//omitted
img.Save(<path to new file>, ImageFormat.Png);

I've verified no matter if I do or do not make any modification, simply reading and saving the image will cause it size to change. Furthermore, if I opened the saved file with Paint and save from there, the file will shrink back to original size.

How do I read and save the image without changing its size?

Upvotes: 1

Views: 1134

Answers (3)

dragon66
dragon66

Reputation: 2715

Apart from the color depth and how many channels (w/o alpha) are used, saved PNG file size depends mainly on two factors:

  • How the pre-processing on image lines (called filtering) is done.
  • The compression level for the deflate algorithm (0-9).

This two factors will greatly affect the output image file size. Filtering is empirical and you can use one out of 4 filtering algorithm for all image lines or different algorithms for different lines or even adaptively try different algorithms on individual lines and choose the largest compression rate. The adaptively way is the most time consuming and impractical for most image writers.

After the filtering, image data is deflate compressed. The compression level for deflate algorithm usually ranges from 0-9 from lowerest to highest compression rate. The higher the compression rate, the slower the compression process. Usually 4 is the best for most of the images.

The filtering process plays a very important sometimes crucial role in PNG compression process. Different filtering algorithm may result in large difference in saved image size. On the other hand, image size is less sensitive to compression level.

You can use tools like TweakPNG to check about the color depth and number of channels the image contains. If the original and the re-saved image has the same color depth and channels, then most probably the filtering and compression level are the culprit for the increased file size.

The truth is if the encoder is not optimized, more often than not, the file size will increase. There are however a lot PNG optimization softwares out there if you don't mind post-processing your resulting images.

Upvotes: 1

Joel Legaspi Enriquez
Joel Legaspi Enriquez

Reputation: 1236

Have you tried playing with the Endoder.ColorDepth field? PNG also supports transparency and might be saving some information not needed by your image.

ImageCodecInfo pngCodec = ImageCodecInfo.GetImageEncoders().Where(codec => codec.FormatID.Equals(ImageFormat.Png.Guid)).FirstOrDefault();
if (pngCodec != null)
{
    EncoderParameters parameters = new EncoderParameters();
    parameters.Param[0] = new EncoderParameter(Encoder.ColorDepth, 24); //8, 16, 24, 32 (base on your format)
    image.Save(stream, pngCodec, parameters);
}

Additional info here: https://msdn.microsoft.com/en-us/library/system.drawing.imaging.encoder.colordepth(v=vs.110).aspx

Upvotes: 0

black dragon
black dragon

Reputation: 3

I think you are missing the compression part.
Add to your code like this -

Bitmap img = new Bitmap(<path to file>);

here is what you missed -

ImageCodecInfo myImageCodecInfo = GetEncoderInfo("image/jpeg");
EncoderParameter myEncoderParameter = new EncoderParameter(Encoder.Quality, 25L);
EncoderParameters myEncoderParameters.Param[0] = myEncoderParameter;

and save like this -

img.Save(<path to file>, myImageCodecInfo, myEncoderParameters);

Here is the MSDN link. hope it helps.

Upvotes: 0

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